The present invention relates to apparatus for controlling the speed of an internal combustion engine, and more particularly to engine speed control apparatus for optimally controlling the engine speed in accordance with the modern control theory.
Such an engine speed control apparatus based on the modern control theory is provided with an observer for estimating an internal state on the basis of control input and output information, the control input being the intake air quantity or the ignition timing and the control output being the engine speed (engine idle speed) and the air fuel ratio as disclosed in the Japanese Patent Publication No. 63-25175. On the other hand, although representing the internal state of a speed controlling system by state variables to determine a necessary control input (controlled quantity of the intake air), the modern control provides a problem that the state variables are required to be obtained by constructing an observer and the construction of the observer takes much time and much trouble because requiring to select adequate parameters and the like by a number of simulations. In addition, since the observer is constructed in correspondence with a dynamic model of a controlled object, the observation of the state variables depends upon only the accuracy in constructing the dynamic model of the speed control system and hence, for heightening the observation accuracy of the state variables due to the observer, the modeling of the controlled object is required to be effected with a high accuracy whereby the model becomes complicated so as to make it difficult to perform the control in practice because of enlarging the load in the control calculations and the like. Thus, the Japanese Patent provisional Publication No. 64-8336 (corresponding to the U.S. Pat. No. 4,785,780) discloses an idle speed control apparatus which controls the engine idle speed by controlling the intake air amount (one control input) without using the observer. There is a problem which arises with such an idle speed control apparatus, however, in that there is a limit in the responsibility due to the delay (dead time) of the air system (surge tank capacity, stroke and others) and the exceeding of this limit makes it difficult to perform the control on the basis of two control inputs including the ignition timing. Moreover, in the case of performing the optimal control on the basis of the multi-input (plural kinds of control amounts) system in accordance with the conventional technique, the calculation amount is extremely increased so as to make it difficult to use the current vehicle computer which has a limit in the calculation speed.